1. Field of the Invention
This invention pertains to thread configurations employed in tubular products such as oil well tubing, casing, line pipe and drill pipe, all of which are referred to as "pipe" for convenience.
2. Description of the Prior Art
Generally speaking, a pipe connection comprises a pin member ("pin") and a box member ("box"). In some cases the box is included in a coupling wherein each of its two box ends provides for joining two pipe joints pin-end-to-pin-end. However, a pipe connection usually joins the pin of a first joint to the box of an adjoining second joint. In the discussion herein, reference will be made to such a usual connection, as a matter of convenience, although it will be understood that a "connection" refers to a connection employed a coupling, as well.
Over the years, pipe connections have been developed which fall into a series of definable categories. The American Petroleum Institute (API) has standardized a single step tapered connection which has an interference thread.
Premium pipe thread construction was introduced with the advent of the two-step thread design pioneered and developed by the Hydril Company. The design is generally known in the industry as the Hydril thread, and is described in U.S. Pat. Nos. 2,532,632 and 2,992,019. Fairly recent improvements in two-step thread designs are shown, for example, in U.S. Pat. Nos. 4,161,332 and 4,192,533. The Hydril two-step thread design was the first thread specifically developed for the increasingly difficult environmental factors encountered with deeper wells.
Various modifications of the API standard thread and the Hydril premium thread have been developed over the years to meet the continually changing needs of the petroleum industry.
A two-step thread design indicates that there is a first thread set generally at a first and smaller diameter and a second thread set axially displaced behind the first thread set generally at a second and larger diameter. Considering all types of pipe, pin ends of pipe can either be tapered or non-tapered, and the threads located thereon can also be tapered or non-tapered independently of the pipe. Non-tapered threads are parallel to the pipe axis throughout the thread set. Tapered thread sets are inclined at the angle intersecting the pipe axis. Tapered thread sets may include roots and crests which are parallel to the pipe axis, parallel to the inclined taper or a combination. In a tapered thread set, the diameter of a thread set does not remain constant throughout its length, such as with a cylinder, but gradually becomes larger progressing from the front end of the thread set. That is, in tracking the diameter of a root or a crest, the diameter becomes increasingly larger along a helical path following the thread set.
In addition, thread sets are designed to be free-running (or clearance) or interference fit. In a free-running design, the mated thread sets on a pin and box can be made up without interference, with clearance between the complimentary roots, crests and thread flanks. Interference threads actually engage upon mating and provide resistance during makeup. The API thread is an interference design, whereas the Hydril non-tapered two-step premium thread is free-running.
API interference threads usually rely on the engagement of the mated threads to provide both the torque stop and the primary seal of the connection. Free running threads require both a torque stop (when fully made up) and an independent seal to complete the connection. There are three locations for torque-shoulder action in a two-step thread: at the step between the thread sets; subsequent to or behind the second and larger thread set on the pin; and at the pin nose. The same areas maybe used for fluid seal locations. Hence, there are three possible fluid seal locations.
The present invention is a variation of the Hydril premium thread in that it uses a free-running, two-step thread with torque stops and seals independent of the thread sets. The present invention utilizes enhanced designed combinations to improve tension efficiency, decrease jump out and burst tendencies and improve leak resistance of the connection. This provides a connection with the same pipe body as prior art connections, but having higher tension, burst and pressure ratings.
Therefore, it is a feature of the present invention to provide an improved thread configuration having a two-step thread arrangement with dissimilar thread set lengths to maximize the tension efficiency of the connection made therewith, to maximize the stiffness of the box member and to maximize the mid-seal integrity with respect to internal pressure, while utilizing the pin nose primary seal of known premium connections.
It is another feature of the present invention to provide an improved thread configuration of a two-step thread arrangement having a double torque shoulder to improved break-out torque.
It is another feature of the present invention to provide an improved connection so as to make it relatively insensitive to entrapment of thread lubricants that may tend to block the pressure energy effect on the primary pin nose seal, to provide extra assembly torque while avoiding high compressive loading, to minimize box buckling tendencies and to provide an improved load distribution.
It is still another feature of the present invention to provide an improved onset structure and an improved completion structure to a thread set to maximize the usefulness of the beginning threads and the completion threads.
It is yet another feature of the present invention to provide an improved thread set arrangement in a two-step or two-set thread wherein the angle of taper for the pin member and box member are dissimilar so that the load on the connection is more evenly distributed.
It is still another feature of the present invention to provide an improved thread set arrangement in a two-step thread wherein each thread set includes negative flank threads, there also being a torque shoulder therebetween which torque action is enhanced by the presence of negative flank threads on either side thereof.
It is yet another feature of the present invention to provide an improved arrangement in a tapered thread set having a torque shoulder at either end thereof.
It is still another feature of the present invention to provide an improved two-step thread arrangement wherein there is an optimum inclusion of two seals and two torque shoulders.